A hot rolling installation of the type in question here usually comprises a hot rolling line having a plurality of rolling stands that are passed through successively in the conveying direction of the steel strip to be hot rolled, and a cooling section for intensively cooling the hot rolled steel strip exiting the final rolling stand of the rolling line.
Installations and methods of the type according to the invention are used to roll what is referred to as “heavy plate”, the thickness of which is at least 15 mm. In the conventional production of such thick steel strips, the respective steel strip is rolled thermomechanically in a reversing manner in a four-high stand. However, this rolling operation lasts very much longer than hot rolling in a hot strip mill. It is therefore desirable to also hot roll thick steel strips in a conventional hot rolling installation.
The rolling of flat steel material, which is intended for the production of thick-walled pipelines which have the most stringent requirements placed on their toughness and insensitivity to crack formation, represents a particular challenge. These properties are usually assessed using the results of what is referred to as the “drop weight tear test”, “DWTT” for short. The DWTT is described in provision API 5L3 of the American Petroleum Institute, 3rd Edition, February 1996, in ASTM E436, in DIN EN 10274 of 1999 and in the Stahl-Eisen-Prüfblatt (steel-iron test sheet) SEP 1326. In this test, a test body of defined weight is dropped from a likewise defined height onto a strip-shaped sheet specimen which is provided with a defined groove-like notch on its side facing away from the impacting test body, in the region of the anticipated break, and is placed with its end portions on a respective support. Here, it is generally demanded that, at a particular predetermined temperature, for example −35° C., the ductile break proportion at the thus produced break in the respective specimen is 85% on average.
Attempts have been made to optimize the toughness of thick steel strips, which are required for the production of oil or gas pipelines, by determined hot rolling and cooling strategies. Various examples of these methods are summarized for example in EP 1 038 978 B1. The method first described in EP 1 038 978 B1 itself allows cost-effective production of high-strength hot strip with excellent toughness. To this end, a precursor material, such as slabs, thin slabs or cast strip, is produced from unalloyed or low alloy steel with additions of micro-alloying elements and subsequently runs through a finishing line formed from a plurality of rolling stands. The precursor material is in this case introduced into the first rolling stand of the finishing line at a temperature which is at least 30° C. above the recrystallization stop temperature of the particular steel. Continuous hot rolling of the precursor strip to form a hot strip is then carried out in one or more passes. The hot rolling is in this case carried out in a temperature range which includes the recrystallization range of austenite. Between two rolling stands, cooling of the hot strip to a temperature which is at least 20° C. below the recrystallization stop temperature then takes place by means of a cooling device, wherein the cooling rate of the cooling is at least 10° C./s. Then, the rolling is continued below the recrystallization stop temperature at a degree of overall deformation of at least 30% in the temperature range below the recrystallization stop temperature, until the finished hot strip exits the hot rolling line.
As likewise explained in EP 1 038 978 B1, steels for the production of thick-walled pipes typically consist of an alloy in which, in addition to iron and unavoidable impurities, (in % by weight) C: ≤0.18%, Si: ≤1.5%, Mn: ≤2.5%, P: 0.005-0.1%, S: ≤0.03%, N: ≤0.02%, Cr: ≤0.5%, Cu: ≤0.5%, Ni: ≤0.5%, Mo: ≤0.5%, Al ≤2%, and up to a total of 0.3% of one or more of the elements B, Nb, Ti, V, Zr and Ca are present. These steels also include the steel grades known under the designations “X70” and “X80”.
Practical experience has shown that, in spite of the measures, in each case of comparable complexity, which are required for the temperature control required in each case in the prior art, although thick hot strips with increased strength can be produced with the methods known from practice, these hot strips do not meet the requirements set in terms of toughness in the field of pipeline construction with the necessary reliability.